Topband: Impedance Bridges

[Donald L Kerouac]

GR Bridges were standard equipment for anyone working on AM broadcast
antennas. They are often still used. Probably the most commonly used
instrument today is the Delta OIB-1 or OIB-3 Operating Impedance Bridges.

Portable, accurate and easy to use, they also require a source of
excitation. They have a built in detector, though it is common to use a
more sensitive external detector.
They can be driven by a signal generator or directly by up to a 5KW
transmitter (up 50KW with external adaptor).
Their main claim to fame is the ability to measure complex impedances in
a directional antenna system where the "cold" Z is very different from
the "hot" Z due to mutual coupling in the array.
I had a problem matching my 160 tower due to mutual coupling to a second
tower only 36 ft. away. My MFJ analyzer said everything was fine, as
would a GR Bridge. The Delta OIB gave very different measurements.
After I detuned the second tower, the MFJ and the OIB agreed.
Since many hams use more than one tower, they may encounter similiar
problems.
73...Don, K9NR

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Post by Donald L Kerouac
I had a problem matching my 160 tower due to mutual coupling to a second
tower only 36 ft. away. My MFJ analyzer said everything was fine, as
would a GR Bridge. The Delta OIB gave very different measurements.
After I detuned the second tower, the MFJ and the OIB agreed.
Since many hams use more than one tower, they may encounter similiar
problems.

One problem that I have noticed using the OIB's in the field is that they
tend to "pull" a bit when you have high reactance values. A GR, as far as I
have seen, does not seem to exhibit this problem. For anyone using a GR
bridge, however, be sure to use not more than about 1 watt from you signal
source on the generator input. Use of higher power will destroy the small
resistor that is at the inpurt of the bridge ( I have done it). I simply use
my MFJ-259 as a signal source and either a Potomac FIM-41 field meter or an
HF rig as the detector.

73................de Goose, W8AV


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[Joe Craig]

I built a small wheatstone bridge with a composition vairable resistor.
When the nul was reached, the resistor was put across a dmm via a DPDT
switch. I used this to prepare a plot of R vs Number of radials.

The reactance was determined by tapping an inductor in series with the
tower for the best nul.

It seemed to give resonable results although I seem to remember the
reactance also varied with the number of radials.

Joe VO1NA